Fixed #21164 -- Added documentation for issue with test users.

The package renaming restores the older package names (which were also the
documented package names). This doesn't affect test discovery because the
module in question doesn't contain any tests.

Thanks to Carl for the design discussion.

docs/topics/auth/customizing.txt

@@ -855,12 +855,14 @@ that the application works with *any* user model, not just the default User
 model. To assist with this, Django provides two substitute user models that
 can be used in test suites:
 
-* ``django.contrib.auth.tests.custom_user.CustomUser``, a custom user
-  model that uses an ``email`` field as the username, and has a basic
+.. class:: tests.custom_user.CustomUser
+
+  A custom user model that uses an ``email`` field as the username, and has a basic
   admin-compliant permissions setup
 
-* ``django.contrib.auth.tests.custom_user.ExtensionUser``, a custom
-  user model that extends ``django.contrib.auth.models.AbstractUser``,
+.. class:: tests.custom_user.ExtensionUser
+
+  A custom user model that extends ``django.contrib.auth.models.AbstractUser``,
   adding a ``date_of_birth`` field.
 
 You can then use the ``@override_settings`` decorator to make that test run
@@ -869,6 +871,7 @@ would test three possible User models -- the default, plus the two User
 models provided by ``auth`` app::
 
     from django.contrib.auth.tests.utils import skipIfCustomUser
+    from django.contrib.auth.tests.custom_user import CustomUser, ExtensionUser
     from django.test import TestCase, override_settings
 
 
@@ -888,6 +891,9 @@ models provided by ``auth`` app::
             "Run tests for a simple extension of the built-in User."
             self.assertSomething()
 
+.. versionchanged:: 1.6
+
+    In Django 1.5, it wasn't necessary to explicitly import the test User models.
 
 A full example
 --------------